How to keep projects of previous blender versions when installing new versions?
First off, reinstalling Blender won't delete any blend file.
Second, there is a portable version for each release, which means you don't necessarily have to uninstall anything and can have multiple versions at the same time. Moreover, the 2.79 RC1 only has a portable version, no installer yet. So in fact you can't even replace your 2.78 installation.

To use the portable version, unzip it and open Blender.exe, very simply.